Lubricant oil formulations generally contain polymeric Viscosity Index (“VI”) improving components which modify the rheological behavior to increase the lubricant viscosity and promote a more constant viscosity over the range of temperatures over which the lubricant is used in, for example, automotive engines. Ethylene-alpha-olefin copolymers (OCPs) have been used to increase viscosity at elevated temperatures. Lengths of ethylene derived units are believed to be instrumental. To maintain the OCP in solution, amounts of propylene-derived units are incorporated into the polymer chain to hinder crystallization of the OCP. Higher ethylene-content copolymers efficiently promote oil thickening, shear stability and low temperature viscometrics, while lower ethylene-content copolymers are added for the purpose of lowering the oil pour point or the co-crystallization with a wax component of the oil.
It is known that narrow molecular weight distribution is desirable for good shear stability and to avoid the inclusion of polymer chains that are either to long or too short to have the desired viscometric effect. Presence of long chain branches has been believed to be undesirable given the potential effect of broadening the molecular weight distribution.
It is known that certain polymerization techniques and polymerization catalysts can be combined to provide levels of detectable long chain branching (LCB), it is also known that diene and free radical modification or polymers can introduce LCB (see WO 99/10422). LCB is defined herein as any branch formed in the polymer that is not derived from the short chain branching (SCB) due to comonomer incorporation. Thus LCB excludes the methyl branching due to propylene insertion or the hexyl branching due to octene-1 insertion. 13C NMR cannot determine the overall length of the LCB chain. However while SCB impacts density and crystallization behavior, by itself SCB does not influence viscous flow behavior which is substantially Newtonian. Non-SCB branching generally leads to distortion of viscous flow behavior, which can be detected by a variety of techniques, including rheology measurements, shear sensitivity under different shear stresses as in MI ratios; internal energy of activation of flow etc. The presence of LCB may also become apparent from other aspects of the molten polymer mass: melt tension, die swell, and melt strength. Further, the presence of LCB may be determined from comparisons of behavior when dissolved in a solvent to determine viscosity as such or the molecular weight in a GPC test. Examples of suitable polymerization techniques are provided in EP 495 099 and EP 608 369.
U.S. Pat. No. 5,151,204 discloses the use of metallocenes in preparing Viscosity Index Improvers but there is no indication of the presence of or level of LCB. EP 632 066 uses a specific metallocene catalyst and the presence of LCB is not disclosed. WO 02/46251 discloses the use of series reactors to produce reactor blends with improved storage flexibility. An oleaginous composition containing a viscosity modifying amount of a linear ethylene polymer which has    (a) melt flow ratio I10/I2 at least 5.63,    (b) mol. wt. distribution Mw/Mn defined by Mw/Mn≧(I10/I2)−4.63 and    (c) a critical shear rate at onset of surface melt fracture of at least 50% greater than the critical shear rate at onset of surface melt fracture of a linear olefin polymer having a similar I2 and Mw/Mn is disclosed in WO 97/32946.
The parameters (a) to (c) are indicative of the presence of LCB. WO9732946 does not exemplify the use of propylene-derived polymers and the effect of improving soot dispersion.
All the publications mentioned herein are incorporated for US legal purposes.
It is among the objects of the invention to provide an OCP which not only has a viscosity modifying effect but also helps to improve soot dispersion in the vehicle crankcase oil.